Various types of web controls for unwinding or unreeling apparatus are known in which a brake is provided braking the roll or reel from which the web is unwound, to maintain tension of the web as it is being passed to a web handling machine, typically a rotary web printing machine. Sometimes the web may tear. Printing machines and other web handling machines have sensors which sense the tension of the web as it passes to or through the web handling apparatus typically a printing machine, and, if a tear is sensed, provide an emergency stop signal which is processed to cause the roll to stop unreeling. Paper rolls, for example rolls used to provide newsprints, especially when full, are very heavy and the inertia of the turning roll is considerable.
German Patent No. 30 44 462 discloses an arrangement in which a support frame carries two web rolls. The web rolls are held on roll cores which are coupled to a core brake. In addition to the core brake, further brakes are provided which are controlled if a web tear should be sensed. This requires complex control arrangements and additional brake structures. Yet, it is not possible to obtain fast or emergency stopping of the web under tear or other emergency conditions, with the braking being carried out under controlled braking conditions. The brakes which are used for emergency or fast stopping conditions are electrodynamic brakes. Generators which, under braking conditions operate as dynamic brakes, are effective but do not act rapidly enough for most emergency braking conditions. Thus, the response time of the brakes of the disclosed system is comparatively long, both under normal operating conditions and, especially, under emergency conditions.
European Patent No. 00 13 368 discloses a system to obtain accelerated stopping under emergency conditions which is matched to then pertaining operating conditions. The arrangement, however, requires a complex control circuitry which utilizes a plurality of choking or throttling arrangements. Such choking and throttling arrangements cause damping of the overall system. If the reference values with which the system operates have a tendency to vary, the control loop will be subjected to substantial dead-time intervals, so that the overall reaction of the system is slow.